1. Field of the Invention
The present invention relates to a liquid crystal display device having a reflective liquid crystal display element, such as a reflective liquid crystal display device and a transflective liquid crystal display device including a reflective liquid crystal display element and a transmissive liquid crystal display element.
2. Description of Related Art
Being thin, lightweight, and low power consumption, a liquid crystal display device is widely used as a portable device display. Since the liquid crystal display device is not a light emitting element, it requires an external light source. Depending on the type of the external light source, the liquid crystal display device is divided roughly into two categories: transmissive-type and reflective-type.
A transmissive liquid crystal display device has a backlight using an light emitting element such as a cold-cathode fluorescent lamp and LED mounted behind the device, that is, the opposite to the side of an observer. The light from the backlight is modulated by a liquid crystal panel, thereby displaying images. On the other hand, a reflective liquid crystal display device uses a light source on the observer""s side such as sunlight as the external light source. The light is reflected by a reflector mounted opposite to the observer, and the reflected light coming back to the observer""s side is modulated by a liquid crystal panel, thereby displaying images.
However, the transmissive type has the problem of dim display in bright ambient light, and the reflective type has the problem of dark display in low ambient light. As a solution to the above problem, a xe2x80x9ctransflectivexe2x80x9d liquid crystal display device having display areas of both reflective-type and transmissive-type in one pixel area has been proposed. The transflective liquid crystal display device is described in Japanese Patent Application Laid-Open No. 2000-187220, for example. The transflective liquid crystal display device serves as a reflective-type with backlight off in bright ambient light to provide brighter images and reduce power consumption, while serves as a transmissive-type with the backlight on in low ambient light to provide brighter and higher-quality images. Therefore, the transflective liquid crystal display device is widely used as a cellular phone display.
FIG. 1 shows a configuration example of the transflective liquid crystal display device described in Japanese Patent Application Laid-Open No. 2000-187220. As shown in FIG. 1, the transflective liquid crystal display device has a circular polarizer 11 and a substrate 21a above liquid crystal layers 22 and 23. The circular polarizer 11 is a composite structures consisting of a polarizer 1, a retarder 2, and a retarder 3. The retarder 3 produces approximately half the retardation of the retarder 2. The substrate 21a has a transparent electrode to apply a voltage to the liquid crystal layers.
The liquid crystal layers 22 and 23 are sandwiched between substrates 21a and 21b, having different retardations. A reflective section further has a reflector 31 reflecting light in a visible light range. Between the reflector 31 and the substrate 21b is an organic layer. A transmissive section, on the other hand, has a transparent electrode to applying a voltage to the liquid crystal layer on the substrate 21b. 
Below the liquid crystal layers 22 and 23 are provided the substrate 21b and a circular polarizer 12. The circular polarizer 12 is a composite structure consisting of a polarizer 4, a retarder 5, and a retarder 6. The retarder 6 produces approximately half the retardation of the retarder 5.
As described above, the transflective liquid crystal display device has a reflective liquid crystal display element in the reflective section, and a transmissive liquid crystal display element in the transmissive section, both in one pixel area. The reflective liquid crystal display element is configured in the order of the circular polarizer 11 (polarizer 1, retarder 2, and retarder 3), substrate 21a, liquid crystal layer 22, reflector 31, and substrate 21b, or in the order of the circular polarizer 11 (polarizer 1, retarder 2, and retarder 3), substrate 21a, liquid crystal layer 22, substrate 21b, and reflector 31. On the other hand, the transmissive liquid crystal element is configured in the order of the circular polarizer 11 (polarizer 1, retarder 2, retarder 3), substrate 21a, liquid crystal layer 23, substrate 21b, and circular polarizer 12 (retarder 6, retarder 5, and retarder 4).
When the angle between a transmission axis of the polarizer 1 and a slow axis of the retarder 2 is xcex81, the angle between the slow axis of the retarder 2 and a slow axis of the retarder 3 is xcex81+45 degrees. Further, when the retarder 2 produces approximately half wavelength of retardation, and the retarder 3 does approximately quarter wavelength of retardation, the composite of the polarizer 1, retarder 2, and retarder 3 serves as the circular polarizer 11 for the subject wavelength. The subject wavelength is generally 550 nm. The retarder producing half wavelength of retardation is called a half wave retarder, and the retarder producing quarter wavelength of retardation, a quarter wave retarder. Similarly, the composite of the polarizer 4, retarder 5, and the retarder 6 serves as the circular polarizer 12. The multilayer circular polarizers 11 and 12 composed of the half wave retarder and quarter wave retarder are especially called wide-band circular polarizers since circularly polarized light can be obtained in a wide wavelength range.
A coordinate system will be explained hereinbelow with reference to FIG. 2 to clarify the directions of the polarizer and retarder. The following descriptions use a right-handed coordinate system where the direction from the backlight toward the polarizer 1 is the positive direction of z-axis. When a transmission axis of the polarizer 1 is xcex11, a slow axis of the retarder 2 is xcex12, and a slow axis of the retarder 3 is xcex13, the axis of the polarizer, half wave retarder, and quarter wave retarder constituting the wide-band circular polarizer have the relation: xcex81=(xcex12xe2x88x92xcex11) and (xcex13xe2x88x92xcex12)=(xcex81+45 degrees).
FIG. 3 schematically illustrates display principle of a transflective liquid crystal display device using a circular polarizer. First, a white display with high reflectivity and transmissivity will be explained. In the reflective section, the liquid crystal layer 22 has such a thickness as to produce quarter wavelength of retardation. In this configuration, as the circularly polarized light having passed through the circular polarizer 11 enters the liquid crystal layer 22, it is converted to linearly polarized light just before the reflector 31. After reflected by the reflector 31, the light returns, passing through the liquid crystal layer 22 again, changing into the circularly polarized light having a different chirality. Therefore, the reflected light can pass through the circular polarizer 11, achieving a bright white display.
In the transmissive section, on the other hand, the liquid crystal layer 23 has such a thickness as to produce half wavelength of effective retardation. In this configuration, as the circularly polarized light having passed through the circular polarizer 12 enters the liquid crystal layer 23, it is converted to the circularly polarized light having a different chirality just before the substrate 21a. Therefore, the light having passed through the liquid crystal layer can pass through the circular polarizer 11, achieving a bright white display.
If a voltage is applied to the liquid crystal layer, liquid crystal molecules stand up, reducing effective retardation of the liquid crystal layer. For simplifying the principle explanation, the effective retardation of the liquid crystal layer is assumed to be 0 when a sufficiently high voltage is applied to the reflective section and the transmissive section. Then, in the reflective section, the circularly polarized light having passed though the circular polarizer 11 enters the liquid crystal layer 22, and it is reflected without any change; therefore, the reflected light cannot pass though the circular polarizer 11, making a black display. In the transmissive section, similarly, the circularly polarized light having passed though the circular polarizer 12 enters the liquid crystal layer 23 to simply pass it through without any change; therefore, the light cannot pass through the circular polarizer 11, making a black display.
As explained in the foregoing, separate setting of the retardations of the liquid crystal layers in the reflective section and the transmissive section allows the both sections to operate in a normally white mode. Also, application of a voltage enables the reflective and transmissive sections to provide a black display; thereby achieving high contrast (luminance ratio) images. Retardations of the liquid crystal layers in the reflective and transmissive sections are adjusted generally by way of adjusting the thickness of the liquid crystal layers in the reflective and transmissive sections.
In the case of using reflected light for an image display, if the reflector 31 has a flat surface, the reflected light at the surface of the display device which is the upper surface of the polarizer 1 in FIG. 1 is overlapped with the reflected light having passed through the liquid crystal layer. It significantly decreases display quality, causing such a problem that black does not appear real black due to the surface reflected light. For the above reason, the reflector is formed to have an uneven surface to scatter light. Generally, the organic layer under the reflector has an uneven surface to make the reflector surface uneven. By adjusting the thickness of the organic layer, the thickness of the liquid crystal layers in the reflective and transmissive sections are adjusted accordingly.
Although it is possible to provide a member such as a scattering adhesive layer to scatter light, on the outer side of the substrates 21a and 21b, it causes problems of blur display and color mixture because the substrates 21a and 21b are as thick as 500 to 700 xcexcm. Therefore, the reflector having a scattering function is generally provided between the substrates. For the same reason, it is also undesirable to provide the reflector 31 outside of the substrate 21b. 
Obtaining high contrast images requires sufficient darkness in a black display. Since human visibility is at its highest for the light having 550 nm wavelength, the subject wavelength is generally 550 nm, and various parameters are set to produce low reflectivity and low transmissivity at the 550 nm wavelength in a black display. Therefore, the present specification uses the wavelength of 550 nm for the value of optical property including retardation. The use of the wide-band circular polarizer allows reduction of reflectivity and transmissivity not only for the wavelength of 550 nm but also for the wavelength close thereto, reducing luminosity of the black display and producing high contrast images.
The liquid crystal layer generally has the twist alignment where a twist angle which is the angle difference in alignment directions of liquid crystal molecules at the interfaces of the both substrates is approximately 60 to 75 degrees, for the following reason. If the twist angle is 70 degrees, there is a gap range where a change in the thickness of the liquid crystal layer, which is called a xe2x80x9cgapxe2x80x9d because it corresponds to the gap width between the substrates, does not affect effective retardation of the liquid crystal layer in the reflective section (S. Stallinga xe2x80x9cJ. Appl. Phys.xe2x80x9d: Vol.86, p.4756, 1999). Within the gap range, the reflectivity does not change relative to the gap width, allowing a wide process margin relative to the gap.
FIG. 4 shows an evaluation result of the dependence of luminance reflectivity of a white display in the reflective section at the twist angle of 70 degrees, on the product xcex94nd of the birefringence xcex94n of liquid crystal material of the liquid crystal layer 22 in the reflective section and the liquid crystal layer thickness (gap) d. In the graph of FIG. 4, the vertical axis is the luminance reflectivity and the horizontal axis is xcex94nd. The luminance reflectivity is the amount that the luminance of reflected light is standardized by the luminance of a light source. The graph in FIG. 4 indicates that if xcex94nd of the liquid crystal layer 22 in the reflective section is approximately 240 nm and above, the reflectivity is constant at approximately 0.34.
In the transflective liquid crystal display element, the transmissive section and the reflective section generally have the same twist angle of the liquid crystal layer and the common circular polarizer 11. Though the use of multi-domain technique allows separate twist angle setting for the transmissive section and reflective section, it costs a lot and therefore the same setting is generally applied. The relation between a twist angle of the liquid crystal layer and xcex94nd (the product of birefringence xcex94n and gap d) to produce high reflectivity and transmissivity in a given twist angle on the condition that the twist angle of the liquid crystal layer are the same and the circular polarizer 11 is common in the transmissive and reflective sections is described in Japanese Patent Application Laid-Open No. 2000-187220, for example.
Although a wide-band circular polarizer consisting of a half wave retarder and a quarter wave retarder is generally used, it is also possible to simply make a circular polarizer by placing a quarter wave retarder so that its slow axis is making a 45 degree angle with respect to the transmission axis of the polarizer. Elimination of the half wave retarder results in reduction in thickness and costs. The reduction in thickness is especially a great advantage for use in portable devices. However, the half wave retarder is generally provided in spite of the disadvantages of higher thickness and costs since it achieves high contrast images. Japanese Patent Application Laid-Open No. 2000-187220 refers to another advantage of preventing display from being colored, though no data for evaluation are presented.
It is assumed in the above principle explanation that effective retardation of the liquid crystal layer is 0 when a sufficiently high voltage is applied to the reflective section and the transmissive section. However, since the voltage is finite in practice, the effective retardation of the liquid crystal layer never becomes 0. Therefore, when using a circular polarizer having a composite structure consisting of a half wave retarder and quarter wave retarder, an actual driving voltage of 5V or below does not provide sufficient darkness in a black display, thus reducing contrast.
Japanese Patent Application Laid-Open No. H11-311784 describes that the above problem can be solved by setting the slow axis of the retarder 3 parallel to a bisector of alignment directions of liquid crystal material at substrate interfaces, and producing less than quarter wavelength of retardation. However, the above prior art does not specify a display color.
In a reflective liquid crystal element using the circular polarizer, if xcex94nd of the liquid crystal layer in the reflective section is in the range of high reflectivity, a white display in the reflective section turns yellow, even when the half wave retarder is used, the slow axis of the retarder 3 is parallel to the bisector of alignment directions of liquid crystal material at substrate interfaces, and retardation is less than quarter wavelength. It is noted that the circular polarizer is an elliptic polarizer to be exact since the quarter wave retarder does not produce exactly quarter wavelength of retardation. In the following, the unit referred to as a circular polarizer can be an elliptic polarizer in the strict sense.
FIG. 5 shows an evaluation result of the dependence of color coordinates (the Yxy color system of CIE1931) in a white display in the reflective section on xcex94nd of the liquid crystal layer in the reflective section. C light source is used. The retarder 3 produces 120 nm of retardation, its slow axis is parallel to the bisector of alignment directions of liquid crystal material at substrate interfaces, and its twist angle is 70 degrees. In the high reflectivity range where xcex94nd of the liquid crystal layer in the reflective section is approximately 250 nm and above, x greater than 0.32 and y greater than 0.33. A measure of the range where color is substantially invisible is 0.285 less than x less than 0.32 and 0.3 less than y less than 0.325, and it is actually yellow compared to white (the C light source, (x, y)=(0.310, 0.316)).
The color of the reflective section depends on the color of the light source. The C light source (correlated color temperature 6774K) used is equivalent to xe2x80x9cdaylight in a blue skyxe2x80x9d. Normal fluorescent light (interior light) has the same or less color temperature. Therefore, when it appears yellow with the C light source, it appears the same under normal use conditions, which is of a problem.
Since the problem that a white display in the reflective section turns yellow occurs under the conditions in the reflective section only, the same problem occurs in a reflective liquid crystal display device using the reflective display element only, and a transflective liquid crystal display device.
The present invention has been accomplished to solve the above problems and an object of the present invention is thus to provide a liquid crystal display device having a reflective liquid crystal display element presenting high quality images with high contrast and substantially white display.
A liquid crystal display device according to the present invention is a liquid crystal display device having a reflective liquid crystal display element including a set of substrates having an electrode to apply a voltage to a liquid crystal layer, a twisted liquid crystal layer interposed between the substrates, an elliptic polarizer consisting of a polarizer, a first retarder, and a second retarder having a slow axis substantially parallel to a bisector of alignment directions of liquid crystal material at substrate interfaces and producing retardation of not more than 140 nm, and a reflector reflecting light in a visible light range, wherein a product (nm) of birefringence xcex94n of liquid crystal material used in the liquid crystal layer and a thickness d of the liquid crystal layer is not less than 120 nm, and not more than a value given by Formula 1:   89.2  +            1.42      xc3x97              10        6                            [                  Re          +                                    (                                                Re                  xe2x80x2                                -                270                            )                        ·                          cos              ⁡                              (                                  2                  ⁢                                      "LeftBracketingBar"                                                                  α                        3                                            -                                              α                        2                                                              "RightBracketingBar"                                                  )                                                    ]            2      
when a twist angle in the liquid crystal layer is in a range of 63 to 77 degrees, an angle between a transmission axis xcex11 of the polarizer and a slow axis xcex12 of the first retarder is in a range of 5 to 25 degrees or in a range of 95 to 115 degrees, an angle between the slow axis xcex12 of the first retarder and a slow axis xcex13 of the second retarder is in a range of 50 to 70 degrees, retardation produced by the second retarder is Re (nm), and retardation produced by the first retarder is Rexe2x80x2 (nm).
In the above configuration, it is possible to obtain a liquid crystal display device providing high quality images with high contrast and substantially white display in the reflective section.
The twist angle in the liquid crystal layer is preferably in a range of 65 to 75 degrees. Also, it is preferable that the angle between the transmission axis xcex11 of the polarizer and the slow axis xcex12 of the first retarder is in a range of 10 to 20 degrees or in a range of 100 to 110 degrees.
The angle between the slow axis xcex12 of the first retarder and the slow axis xcex13 of the second retarder is preferably in a range of 55 to 65 degrees. Further, the product of birefringence xcex94n of liquid crystal material used in the liquid crystal layer and a thickness d of the liquid crystal layer is preferably not less than 130 nm.
It is further preferable that the liquid crystal display device has a transmissive liquid crystal display element including a second elliptic polarizer consisting of a second polarizer, a third retarder, and a fourth retarder, and a second twisted liquid crystal layer interposed between the substrates, and that a slow axis of the fourth retarder is inclined with respect to the bisector of alignment directions of liquid crystal material at substrate interfaces at xe2x88x9215 to +15 degrees, and retardation produced by the fourth retarder is 80 nm to 140 nm. The above configuration allows a liquid crystal display device providing high quality images with high contrast and substantially white display in the reflective section.
The slow axis of the fourth retarder is preferably inclined with respect to the bisector of alignment directions of liquid crystal material at substrate interfaces at xe2x88x9210 to +10 degrees.
It is preferable that an angle between a transmission axis of the second polarizer and a slow axis of the third retarder is in a range of 63 to 70 degrees or in a range of 153 to 160 degrees, and an angle between the slow axis of the third retarder and a slow axis of the fourth retarder is in a range of 108 to 115 degrees. The above configuration allows a liquid crystal display device providing high quality images with high luminosity (transmissivity) in a black display in the transmissive section. The fourth retarder is preferably a hybrid alignment liquid crystal display film.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.